I have always been a gamer/musician hybrid, and therefor would split my free time between the two. Playing games as a video game sound designer could be called "case studies" or "research" because you're playing something that a group of audio engineers poured their heart into, and got paid a very nice paycheck for it. That being said, games such as Mass Effect 2 and Battlefield Bad Company 2 have outstanding audio work done, and have received numerous awards at GDC and other similar conventions.
As far as which ones games I go out and purchase goes, that's where youtube, and forums come in. I'm not a big fan of 60 dollar console games, and with the sad fact that businesses like blockbuster no longer are around to provide the public with a rental service to try things out before they commit to dropping serious dollars on a product they might end up disliking.
What I like to do, is learn who the sound team on the game is, and research those folks, and read anything that they might have posted on a forum, interview, etc... Because after all, those people are your "competition" and should be a driving force to push you to perfection. You'll usually find some repeated names, which gives me more of an incentive to find out what those guys did when they were my age, looking for work.
Finally, most of the time, new releases have something new to show to industry, including audio. I remember when I played Mass Effect 2 for the first time, and starting using my biotic powers, and started noticing little tricks the team used in order to keep that bassy, multi-band biotic sound sharp and clean while everything else is going on. (I won't divulge that, but it involves a side-chained low-pass filer built into the audio engine)
Anywho, I'm not going to tell you to buy every game you see, instead, try watching youtube videos of it, or on the games homepage for some in game footage, and read read read read everything you can. I know I can't afford a 60 dollar game every time I see something I want to play.
Cheers fellow sound buddy.